The duration and frequency of your practice sessions depends on your level, goals and available time. Beginners can't play as long as conservatory students and someone who wants to become a professional musician will practice more than someone who just wants to play in the band for fun.
So here are a few guide lines for students, parents and teachers. These are for individual practice and don't include band rehearsals.
Beginners: I recommend students who are just starting to do 30 minutes of individual practice 5 times/week on top of the school band. It should be enough to gradually build some chops and learn the basics. If you played another brass instrument before, you already have some chops and should be able to play longer.
2-3 years of playing: Once you have the very basics covered and have developed some chops, you can start increasing the duration of your practicing sessions to 45 minutes depending on what you are playing. You should be doing a mix of scales, etudes, solo pieces and your band parts at this point. You should still be doing about 5 sessions/week but that depends on how much time and motivation you have. It's not always easy to find 5 slots in a weekly schedule when you have home works, school projects, football practice etc. on top of your band rehearsals but if you really love music, you'll find time for it. Make yourself a realistic weekly schedule with all your activities and include practicing sessions in it. You'll probably have to practice a bit less if you have important exams and need to study more on a certain week but you can always catch up later.
4-5 years of playing : If you have been practicing well in your first 3 years, you should have enough chops and technique to do a decent job in your school band by now so your practicing time will depend on your goals. If you just wish to play for fun and aren't playing a high part in the band, the rehearsals will probably be enough to keep your chops going. You might have to put an hour or two per week sometimes to make sure you know your parts well and aren't a drag for the rest of the group though.
If you wish to challenge yourself to take your playing at a higher level, you'll have to play about 60 minutes per day 6 days/week. You can split this in two sessions and do a bit more. Keep doing your scales, etudes and solo pieces to improve your technique and learn the basic repertoire of your instrument.
Conservatory students: At this level, you will have many different things to practice and you'll have to do them well. You need to organize your practicing sessions in a way that allows you to work thoroughly on each and every assignment that you have. So on top of your instrument lesson, chamber music and orchestra rehearsals and other classes, you should be practicing about 2-3 hours per day for horn and trumpet players. Low brass players can do more.
Don't do it all in one session. Ideally, you can do one session in the morning with a break in the middle and another session in the evening. As brass players, we can't play as long as string or piano players so we need to practice efficiently and not waste our strengths. An hour of good and efficient practice is worth more then 5 hours of sloppy and botched work. Right down on a piece of paper everything you do (scales, etudes, daily routine, chops building, trills etc.) and for how long. That way you can keep track of your work and adjust your schedule if needed.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.